Altimetry and Transponder Ground Simulation Experiment · interplanetary link laser pulses are...

Proceedings of the 16th International Workshop on Laser Ranging

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Altimetry and Transponder Ground Simulation Experiment

K.U. Schreiber

a, M. Hiener

a, B. Holzapfel

a, H. Michaelis

b, N. Brandl

c,

K.-H. Haufec

aForschungseinrichtung Satellitengeodaesie, Technische Universitaet Muenchen

Geodaetisches Observatorium Wettzell, 93444 Bad Koetzting, Germany bDLR e.V. Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany

cBundesamt fuer Kartographie und Geodaesie

Geodaetisches Observatorium Wettzell, 93444 Bad Koetzting, Germany

schreiber©fs.wettzell.de

Abstract

We have designed and built a compact demonstrator unit for the investigation of altimetry

and transponder applications. A small light-weight breadboard carries a compact frequency

doubled Nd:YAG pulse laser, afocal beam expansion optics, a small receiver telescope with

spectral and spatial filter arrangements and a photosensitive detection device (SPCM, PMT

or SPAD). The output laser energy can be as high as 45 mJ with a pulse-width of 3 ns and

the telescope aperture is 12 cm. Simulations [1] suggested that the link margin for low Earth

orbiting satellites (LEO) is comfortable and that it may be possible to obtain echoes from a

dual-station experiment in several different configurations. This paper outlines details of the

experiment and presents the obtained results.

Key words: Satellite Laser Ranging, Optical Transponder, Laser Altimetry,

Interplanetary Ranging

PACS: 06.30.Gv, 42.79.Qx, 95.55.-n, 42.50.Ar

1. Introduction

Laser ranging at interplanetary distances has become a viable option in recent times. The

advances made in optical communications as a result of the need of higher transmission

bandwidth for the recent generation of imaging satellites provides the necessary

infrastructure namely pointing and spaceborne telescope design.

Figure 1. A transponder for optical ranging consists of a laser transmitter and a photo-

sensitive receiver on both ends of the distance of interest.

Preprint submitted to ILRS May 9, 2009


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This suggests to include inter-satellite ranging and Earth to interplanetary satellite ranging as

newly available means of precise orbit determination (POD). In other cases of dedicated

space missions for fundamental physics, such as the proposed "Einstein Gravity Explorer"

[2] laser ranging may provide the tools for high-precision clock comparisons. Therefore it is

time to study the potential of interplanetary ranging in more detail. Since dedicated

experiments in space are far to expensive and time consuming to be realized, there are only

two other possibilities left. Either an existing pulse laser equipped mission can be "misused"

to evaluate an interplanetary optical ranging link or a ground based experiment has to be set

up such that all relevant parameters of an interplanetary optical link can be examined in great

detail. Figure 1 shows the basic concept of an optical transponder. There are two transmitters

and receivers located at the ranging terminals A and B. They are firing short laser pulses at

each other asynchronously and from a large distance. While the epochs of the detected

signals are timed on each local computer, the epochs of laser fire are exchanged via telemetry

between both stations. From this one can compute the range and the rate of change (on the

radial component) between both stations as well as the respective clock offset. In this way

the dependence of the receive signal on the covered distance follows a 1/r2 relationship rather

than 1/r4 like in the case of a two-way optical link from backreflections off corner cubes on

artificial satellites and on the moon [3]. While the moon is at about the largest distance that

can be practically tracked by using passive retro-reflectors, transponder have the potential to

work within the entire solar system on a reasonable link margin. Until today there have been

two feasibility studies, which were performed as experiments of opportunity making use of

NASA spaceprobes in orbit around Mars and in transfer to Mercury [4], namely:

One-way ranging to Mars Global Surveyor at a distance of 80 million km

Two-way ranging during a MESSENGER fly-by at a distance of 24 million km

The Mars Orbiter Laser Altimeter (MOLA) was orientated pointing toward Earth in order to

detect laser pulses transmitted from the 1.2 m telescope of the Goddard Space Flight Center

in Greenbelt. Since the transmitter of MOLA is no longer working only one-way ranging

could be demonstrated successfully based on a very small number of unambiguously

identified datations. In a similar way an equipment test of the Mercury Laser Altimeter

(MLA) during a fly-by of the MESSENGER spacecraft was used to determine the range and

clock offset between the spacecraft and the ground station in Greenbelt by a complete

ranging link in both directions. Consistent results were obtained despite the fact that the

number of data points were sparse and the laser pulse width of the altimeter was not ideal for

such a ranging application.

2. Transponder Simulation

In a very general way one can consider satellite laser ranging (SLR) as a special laser

transponder application. Provided that there are two ranging stations located sufficiently

close together (within the divergence angle of the reflected laser beam), it is possible to test

parts or all functions of a transponder link in an entirely ground based experiment. According

to [1] the equivalent transponder distance rt relative to an SLR laser link of the same station

is a function of the actual slant range rs. For a satellite in near circular orbit at the height h

above sea level rs can be expressed as


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where Θ is the zenith distance and rE = 6378 km the mean radius of the Earth. The equivalent

transponder distance then becomes

Figure 2. Example of a Topex pass observed by the WLRS on June 20, 2001 under

cloudy conditions. The returns on the standard ranging configuration are shown on the

lower trace, while the echoes on the small extra receive aperture are displayed on the

upper trace.

One can see that it strongly depends on the slant range and on σs the laser cross-section of the

target in square meters. In the following we are assuming an atmospheric transmission TA of

around 0.7, which is the maximum value for the given operational wavelength of the ranging

systems of λ = 532 nm. It is difficult to establish a reliable value during the measurements.

As pointed out in [1] "real world" tests of transponder functions for distances as far out as the

outer planets of the solar system are possible. In a first test we have mounted a small

telescope with an aperture of 12 cm to the Wettzell Laser Ranging System (WLRS), pointing

along the optical axis of the main telescope in order to detect laser echos independently from

the routine ranging hardware. Over a period of several month echoes from a number of satel-

lites including Lageos and Etalon were recorded in parallel on the additional telescope. One

example of a satellite pass from Topex on June, 20 in 2001 is shown in fig. 2. This was an

intermediate step towards a transponder testbed installation, since it demonstrated a

sufficient link margin for such a small receive signal aperture. At the same time this basic

setup already resembles a simplified version of the basic layout of an asynchronous

transponder concept as outlined in [1]. Figure 3 illustrates the entire operation principle.

There are essentially two timescales involved, one for each terminal of the two stations

separated by a large distance. Under typical conditions it can be assumed that there is an

offset between the two timescales as well as there is a relative drift between them. In the case

of an interplanetary transponder the range r between the two stations would be

(1)

(2)


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Figure 3. Functional diagram of the transponder simulation application. Time is

progressing along the x-axis, while the distance is corresponds to the y-axis. For an

interplanetary link laser pulses are transmitted directly between the two ranging

terminals. In a ground based simulation experiment, geodetic satellites are used to

backreflect the laser pulses, so that they can be detected by an independent ranging

system on the ground near the transmitting station.

where c is the velocity of light and tn,m are the corresponding epoch registrations on each

ranging terminals. During a typical transponder application many such pulses are transmitted

from both sides and a priori range estimates have to be applied to work out the respective

corresponding pairs of epoch registration used in eq. 3. However for a transponder simulation

setup with a geodetic satellite reflector target employed there are essentially three scenarios

available for studies. Case 1 is the usual satellite ranging operation for the station associated

with timescale 1 alone. Case 2 is the additional laser ranging operation associated only with

timescale 2. Transponder operations (case 3) can be realized by transmitting laser pulses

asynchronously from one station, receiving it by the other station and vice versa. In the most

simplified case both stations share the same telescope drive for pointing, while all other

hardware components like timers, control system and lasers etc. are independent. More

generally the two ranging terminals could be realized by two individual SLR stations in close

proximity as proposed in a NASA concept [5].

3.Transponder Demonstrator Experiment

A balanced transponder link is characterized by a comparable number of recorded photo-

electrons on both ends of the transmission path. For an optical satellite to satellite link this

usually would mean the application of similar laser transmitters and telescope apertures on

both terminals. However for a ground to satellite link it is desirable to have a large telescope

and a powerful laser on the ground where it is readily accessible and only a low-power laser

and small telescope in space. This reduces the demands on payload requirements

substantially. Therefore we have followed a similar approach for our testbed experiment. The

WLRS was used as Station A [6]. It operates a 0.75 m diameter telescope and a 180 mJ

frequency doubled Nd:YAG laser. The pulse width is 80 ps. A Quantel Brio laser with 35 mJ

(3 ns pulse width) with a transmit aperture of 3 cm was used for Station B. The

corresponding transmitter divergence has a minimum of 24 arc seconds. The receive

telescope is a 12 cm Maksutov type aperture with additional spatial and spectral filters in

order to allow daylight operation. Several different photo-detectors may be attached.

(3)


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Experiments were carried out with a PMT (12% quantum efficiency), the SPCM module of

EG&G and a SPAD for single photo-electron detection.

Figure 4. Hardware layout of the transponder simulation module. On the left hand side

one can see the laser (from the backside), the transmit and receive aperture. The top

view on the right hand side also shows the filter arrangement behind the receive

aperture and the photo detection device

Figure 4 shows the compact design of the transponder simulation module. A diaphragm in

the receive path allows the adjustment of the receiver field of view in the range of 20 to 90

seconds of arc. Basic test operations of the transponder simulator were carried out by ranging

to a local ground target, namely a diffuse reflecting concrete pillar 180 m away.

Figure 5. Example of a ground target range measurement sequence. The low energy

laser pulse was reflected from a concrete pier at a distance of about 180 m.

Figure 5 shows a typical result of these ground tracks, detected with the photomultiplier as a

receiver. In order to operate the full transponder experiment we have set up a completely

independent ranging environment, including the event timer and control system software as

outlined by the block diagram in fig. 6. For simplicity it is assumed that the computer control

and data logging is part of the timer module. Each system needs a start and stop pulse to

perform the range measurement function and in addition a gate signal to arm the detector for

the reception of the return signal. Since both systems operate entirely asynchronous with a


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repetition rate of 10 Hz in the case of the WLRS and 20 Hz for the transponder module, it is

necessary to cross over both the detector signal cable and the rangegate generator signal

between the two systems. Apart from this change both stations are treated like independent

ranging environments. Following [1] link budget calculations show the expected return

signal strength for a number of configurations summarized in table 1. For this calculation a

zenith distance of 30° and an atmospheric transmission of 0.7 was used. This corresponds to

reasonably good but not ideal conditions and assumes a collimated laser beam for both

transmitters. In order to operate the simulation module on top of the laser telescope of the

WLRS one has first to align the optical axis of the transmitter of the simulation module to be

collinear with the corresponding optical axis of the receiver. This was done by imaging the

ground target 180 m away from the simulator test location on the receive telescope of the

simulation module. Once the laser beam was in the center of the field of view of the receiver

telescope the alignment screws were tightened up. Then the entire base plate was lifted and

mounted on a adjustable frame over the elevation axis of the WLRS telescope. Figure 7

shows the simulation module sitting on the much larger telescope of the WLRS. The

alignment of the simulation module relative to the optical axis of the WLRS was achieved by

pointing the two systems to a reference target approximately 5 km away from the ranging

site. The entire simulator was adjusted with the mounting frame to also image the remote

reference target in the eyepiece of the receiver telescope.

Figure 6. Block diagram illustrating the cable crossover required for the transponder

simulation experiment.

Table 1. Expected link budget for ground based transponder simulation. The figures

for the WLRS are computed for the Micro-Channel-Plate photomultiplier.


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Figure 7. Simulation module mounted on top of the WLRS telescope.

4.Experimental Results

4.1. Transponder Link

The transponder module remained on the WLRS telescope for one week and a total of 21

successful satellite passes were observed in different arrangements, covering all the different

options of table 1. In this paper we only report results from the two most demanding

configurations, namely the actual transponder setup and the case where a high Earth orbiting

satellite (LAGEOS 1) has been observed with the transponder module in SLR configuration.

Figure 8 shows the result for a daylight Beacon C path, asynchronously running in the

transponder configuration (fig. 6). This satellite is in a near circular orbit approximately 1000

km above the ground. According to tab. 1 one would expect a stronger echo detection rate on

the WLRS system, because of a link margin, which is by a factor of two higher and because

of the much higher laser pulse rate. However the data recorded by the transponder hardware

turned out to be much denser in the end. Although the reason for this behavior was not

unambiguously identified, circumstances suggest that this happened because of a residual

misalignment of the emitting laser beams between the two ranging systems. While the

colinearity of the optical axis of both receivers has been verified repeatedly by observing a

landmark several kilometers away, similar checks for the transmitting branches of the two

system have not been possible. Therefore it is very likely that a small misalignment is

responsible for the reduced link budget, because the divergence of the laser beam of the

transponder had to be increased. In subsequent observations there were two regimes

identified around 30 seconds of arc apart, where the return rate for each of the two systems is

at a maximum [7]. After the noise and the trend was removed from the residuals of both data-

sets by fitting a polynomial to the transponder data and applying the same parameters to the

WLRS measurements, the plot of fig. 9 is obtained. The observed scatter of the WLRS data

is much higher than for the simulation unit. This is caused by the much broader pulse width

(3 ns) of the Brio laser. In comparison the WLRS laser provides a pulse width of 80 ps.

There is more noise in the WLRS ranging window (fig. 8 bottom part) because of a repetition

rate twice as high as for the WLRS laser. The effect appears to be smaller than it is in reality,

since the WLRS operates under a much smaller field of view (50 μrad rather than 200 μrad)

and also uses a cooled detector. The transponder system has not been calibrated to the

reference point of the WLRS hence the observed offset of 10 ns between both systems.


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Figure 8. A simultaneous observation of a Beacon C satellite pass with both ranging

systems in asynchronous transponder configuration. The pass was observed on August

16, 2008 in daylight.

Figure 9. The filtered transponder echos from fig. 8. The data recorded on the WLRS

exhibits a larger scatter caused by the 3 ns pulse width of the transponder transmitter.

The offset between the two tracks is introduced for better visibility.

4.2. Biases from Receive Signal Intensity Variations

Apart from demonstrating the basic functions and the feasibility of this transponder link

experiment, this setup is also useful to investigate systematic receive signal biases [8]. For

this purpose only the WLRS laser fires and both telescopes receive the same return signal

from the satellite. According to tab. 1 there is approximately a factor of 50 difference

between the generated numbers of photo-electrons on each detector system. Some of this

imbalance in receive power will usually be compensated by an adjustable gray wedge in the

receiver path of the WLRS system, but there are no simple rules for the settings. However

when the range residuals from a weak and a strong signal return level are plotted against each


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other and intensity dependent detector delays appear, this will reflect as a systematic

signature in the residual vs. residual plot.

Figure 10. Range residual plot of a Beacon C pass taken on August 13, 2008.

Simultaneous detections on both ranging systems are plotted against each other in this

chart.

Figure 10 shows such an example for a observed Beacon C pass. A higher density of

observations suggest higher intensity on the return signal. The linear regression of this data

set yields a slope of 1.008, which is the first indication of a systematic difference between

both signals. In the next step we have applied the linear regression to the data and then

plotted these residuals versus time. This yields the scatter plot of fig. 11. The solid line

superimposed on this data was derived by taking the average of all data points in 1 minute

intervals. As expected, areas of high data density have a tendency towards shorter ranges.

This is in accordance with both the behavior of avalanche photodiodes operated in the Geiger

mode, where a higher return signal level by-passes several stages in the multiplication

process, which leads to the breakdown of the bias voltage of the diode. Also photomultipliers

employing a constant fraction discriminator for a signal voltage independent timing of the

arrival of the laser pulse are expected to measure shorter ranges for higher signal levels, since

the discriminator only works in a very limited signal voltage range.

4.3. Simulated Interplanetary Distance

Finally, we have also explored the weakest satellite link listed in tab. 1 and observed a

LAGEOS satellite pass on the transponder module alone. The return echo rate was very weak

as it is to be expected from a statistically obtained 2 photo-electrons from such a small

system.

Figure 12 shows the measured range residuals. There were about 60 returns detected within

560 seconds. This means that 0.5% of all shots (repetition rate: 20 Hz) obtained a valid return

signal. This is less than expected from tab. 1. A 2 photoelectron return signal level would

usually result in a data rate of about 10%. We believe that this discrepancy can be attributed

to both, a slightly larger beam divergence of the outgoing laser beam and an effective

atmospheric transmission below the maximum value of TA= 0.7. During the time, where

valid satellite returns have been detected the slant range varied from 7500 km ≤ rs ≤ 6700 km

with the marginally higher return rate at the longer distances at the beginning of the pass.

With sr = 7500 km, a radar cross section of the satellite LAGEOS of σs = 15 million square


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meter and an assumed value of TA = 0.6 the equivalent transponder distance for the here

described transponder simulation module turns out to be rt = 0.44 AU.

Figure 11. Range residual plot of a Beacon C pass taken on August 13, 2008. The

residuals are obtained from a linear regression on simultaneous detections on both

ranging systems. The black line was derived from this data set by averaging the data

over 1 minute intervals.

Figure 12. Range residual plot of a LAGEOS 1 pass taken on August 19, 2008 using

the transponder module alone. The satellite track is very weak as expected from the

low signal level calculated for this case (tab. 1).

If we apply the same calculation for the WLRS system, which routinely tracks GPS satellites

and also obtained laser echoes from the Apollo 15 laser reflector, we find a convenient

equivalent transponder distance of rt ≈ 4 AU and a maximum demonstrated marginal

transponder distance of rt ≈ 110 AU. However this would require a balanced transponder

link, with a space segment about 5 times more efficient than the module investigated in this

paper.

5.Conclusion

In this paper we have simulated basic properties of optical interplanetary transponder links

experimentally. A small transponder module comprising a pulse laser, a small telescope


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including provisions for spectral and spatial filtering and an event timer unit was used

together with the Wettzell Laser Ranging System for the simulation. During the experiment

laser pulses from one system were transmitted to orbiting geodetic satellites. The back-

reflections were recorded on the respective other system. The characteristics of the two

independent ranging instruments were then used to compute the equivalent transponder deep

space distance. While the transponder module showed sufficient sensitivity to cover

distances of up to 0.44 AU, the WLRS has a comfortable l-way range margin of about 4 AU.

However, since successful lunar echoes have been obtained from the WLRS in the past, it

has demonstrated a very marginal equivalent transponder range to about 110 AU, but this

cannot be called operational. While the laser link margin is certainly an important parameter,

other needs like precise pointing and the alignment of the transmit and receive path are of

similar concern for successful interplanetary transponder applications. This is also evident in

our experiment, since the measured signal levels always lagged behind the link budget

calculations. The great advantage of the transponder simulation is a complete end to end test

of a transponder link under real conditions, including all of the involved hardware and the

atmosphere, from an easily accessible ground based observatory. The only differences

between this experiment and a real deep space transponder link are a double pass of the

optical signal through the atmosphere and the alignment of the optical path between the

observatory on the ground and an interplanetary spacecraft.

Acknowledgement

This project is supported by Bundesamt für Karographie and Geodäsie (BKG) and the

Deutsches Zentrum für Luft- und Raumfahrt (DLR), Berlin. The authors would like to thank

Wolfgang Schlüter from the Bundesamt für Kartographie und Geodäsie for their support of

the project and Michael Wensauer for the assistance in preparing the necessary

instrumentation.

References

[1] J. J. Degnan, "Simulating Interplanetary Transponder and Laser Communications

Experiments via Dual Station Ranging to SLR Satellites"; Proceedings of the 15th

International Workshop on Laser Ranging, Canberra, Australia

[2] S. Schiller, G.M. Tino, P. Gill, C. Salomon and 34 others; Einstein Gravity Explorer - a

medium-class fundamental physics mission; Exp. Astron., DOI: 10.1007/s10686-008-

9126-5, (2008)

[3] J. J. Degnan; "Asynchronous laser transponders for precise interplanetary ranging and

time transfer‖; J. Geodynamics 34, 551 - 594, (2002)

[4] D. E. Smith, M. T. Zuber, X. Sun, G. A. Newman, J. F. Cavanaugh, J. F.

McGarry, T. W. Zagwodzki; "Two Way Laser Link over Interplanetary Distance";

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[6] W. Schlüter, R. Dassing, P. Sperber, R. Kilger, U. Schreiber; The Role of the

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[7] B. Holzapfel, "Transpondersimulationen für Anwendungen in den Geodätischen

Raumverfahren; Diploma Thesis; Universität der Bundeswehr, München, (2008)

[8] U. Schreiber, A. Schlicht, K.H. Haufe, "Systematic Biases in Laser Range

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